Subscriber lines associated with end user voice grade telephones, two-party divided analog subscriber telephones and coin operated public telephones terminate at a central office terminal by means of what are commonly referred to as line cards. Such lines cards typically provide two-wire loop-start or ground-start service in a carrier serving area environment. Such a line card may be used for plain old telephone subscriber service (POTS) or for loop-start or ground-start private branch exchange (PBX) service.
The signaling states associated with such subscriber lines have previously been detected through use of discrete analog circuitry and then encoded using bit-map detectors as dictated by signaling bus width. For instance, if six detector states are to be detected and if there are four signaling lines in the signaling bus, then encoding of the six possible states must be performed with corresponding decoding at the other end. For designs requiring more signaling states (signaling states are also referred herein as detectors) than the signaling bus width, the added cost and printed circuit board area associated with such encoders can quickly become prohibitive.
The present invention provides an alternative approach to signaling detection and uses a microcomputer in association with an analog to digital (A/D) converter and an analog front-end circuit to reduce the cost and space requirement to implement these signaling detection functions. The actual detection parameters are controlled by firmware residing within a microcomputer.
The detection circuit and routines employed therein can be used to detect virtually any telephony signaling state in a manner which can be easily modified or augmented as different signaling states are desired for purposes of detection. In addition, the detection circuit requires a limited amount of circuit board area for implementation which is independent of the type and number of signaling states to be detected.